The development of smokeless powder in the latter nineteenth century allowed cast lead bullets to be driven at much higher velocities than was possible using black powder. New, high intensity cartridges were developed to make the most of this new propellant and chamber pressure in the barrel of the rifle doubled in some cases.
Up to, and during, this period, cast lead bullets had been made using pure or nearly pure lead. The base of the bullet rested right on the powder on very close to it. The higher pressures of the new smokeless propellants made two changes necessary. A harder bullet metal to withstand the increased velocity and something to protect the base of the lead alloy bullet from the cutting and erosive effects of the smokeless propellant.
As a result, a small cup of copper, or gilding metal, called the "gas check" was developed for use with high intensity cartridges. When a gas check is applied to the base of lead bullets designed to accept it, the gas check provides a substantial degree of protection to the bullet base and allows greater accuracy at the highest velocities generated by the smokeless propellants. Gas checks are currently widely used for this purpose and the use of cast lead alloy bullets has greatly increased in sophistication.
One of the major problems facing the shooter who employs cast bullets is the inability to apply the gas check squarely onto each bullet. A square fit of the gas check onto the bullet base has been found essential for accuracy.
Currently there are two types of gas checks. One type may be of a slip-on type and the other the crimp-on type. There are also a growing number of large and small bullet mold makers with their individual versions of gas check bullet designs.
The shooter who wishes to cast his own bullets and use a gas check design must find some way to squarely apply a gas check of one type or another onto the gas check shank of a bullet cast from a mold. Many gas checks are applied in conjunction with the sizing and lubricating of the cast bullet in devices made for that purpose. Such apparatus is called a lubricator-sizer of which there are a number of different versions available. Generally, the differences in the design specifications for bullets and molds and the normally large manufacturing tolerances that are encountered, make it very difficult to obtain a perfect square fit of a gas check onto a bullet using techniques involving such lubricator sizers. It is, therefore, desirable to provide a technique whereby a gas check can be fit squarely onto a cast bullet in a convenient manner.